Effective Pumping Speed and the Leakage Budget

FROM THE ENGLISH BOOK

A chamber can pass a leak test yet fail to reach its operating pressure. Line conductance, outgassing and seals in the installed configuration may change the gas balance.

Use pumping speed at the chamber

For a pump connected through a line in the applicable steady-flow model, effective pumping speed is given by:

1/S_eff = 1/S_pump + 1/C

S_eff and S_pump are pumping speeds and C is line conductance, all expressed in consistent volume-per-time units. Conductance depends on the gas, geometry and flow regime; use the model appropriate to the operating pressure.

With S_pump = 0.10 m³/s and C = 0.010 m³/s, S_eff is approximately 0.00909 m³/s. Increasing pump speed to 0.30 m³/s raises effective speed to approximately 0.00968 m³/s, only about 6.5%. The connection is the dominant restriction in this example.

Allocate the gas load before setting a leakage limit

For the adopted steady-state pumping model:

Q_leak,lim = S_eff(p_lim − p_ult) − Q_other

Q_leak,lim is permissible leakage throughput in Pa·m³/s; p_lim is the permissible pressure and p_ult the ultimate pressure used in the model, in Pa. Q_other includes the other gas loads in Pa·m³/s. Avoid counting the same gas source both in ultimate pressure and in other gas loads. Use a separate balance for gas components where one effective pumping speed is not justified.

A chamber with limited pumping margin

Let S_eff = 0.10 m³/s, p_lim = 1 × 10⁻⁵ Pa and Q_other = 8 × 10⁻⁷ Pa·m³/s. Neglect ultimate pressure. Total allowable throughput is 1 × 10⁻⁶ Pa·m³/s, leaving only 2 × 10⁻⁷ Pa·m³/s for leaks. Ignoring other sources overstates the leakage allowance by a factor of five.

A negative calculated leakage budget means the pressure target cannot be achieved under that model even with no leakage contribution. Reduce other gas loads or increase effective pumping speed before assigning a feasible limit.

Keep the conclusion within the test conditions

Pumping speed and leakage throughput have different dimensions. Record gas, pressure, temperature, system configuration and detection checks with the result. An absent detector response establishes only what was not detected under the demonstrated test conditions.

Adapted from English book preview · pages 19–31; original pages 125–137. This online article is an edited summary of selected material.